Problem: $ { \left[\begin{array}{rr}{1} & {-1} \\ {-2} & {-2}\end{array}\right]+\left[\begin{array}{rr}{0} & {0} \\ {-1} & {-1}\end{array}\right] = {?} } $
To find the sum of two matrices, add the corresponding items between each matrix. ${\left[\begin{array}{rr}{1}+{0} & {-1}+{0} \\ {-2}+{-1} & {-2}+{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{1} & {-1} \\ {-3} & {-3}\end{array}\right]}$